Pressure-sensitive label stock normally consists of a web of printable label material such as paper, film, foil or a combination of paper, film and/or foil, with an adhesive coated to the bottom side of the web, as well as a silicone-coated release liner covering the adhesive. The label is applied by peeling off the silicone-coated release liner and pressing the label to another surface. The adhesive is inherently tacky and forms a bond to the surface by the application of light pressure. The label can form a permanent bond or a removable bond dependent on the specific adhesive chosen.
Because conventional pressure-sensitive labels use a release liner, pressure-sensitive labels are relatively expensive and generate additional waste as compared to linerless labels. The primary object of the present invention is to eliminate the silicone-coated release liner, thereby resulting in significant waste reduction and printed label cost.
For industrial applications, pressure-sensitive label stock is typically supplied in rolls to label printing facilities. In most applications, the roll is loaded onto the front end of a roll label printing press. The label stock is fed from the roll through one or more printing stations and then one or more die cutting stations to print and cut the labels. In some applications, the continuous web is rewound for storage or shipment to a packaging facility. Sometimes the label stock is cut into sheets and/or individual labels which are stacked in magazines.
As mentioned, the pressure-sensitive label stock normally consists of a web of printable label material such as paper, film, foil or a combination of paper, film and/or foil, with an adhesive coated to the bottom side of the web, as well as a silicone-coated release liner covering the adhesive. The release liner is important to facilitate handling of the label stock and prevent “blocking” of the rolls or stacks of printed or unprinted label stock. In other words, when being stored in a roll or a stack, it is unacceptable for the various layers of the web to stick to one another; otherwise the layers tend to form a block which renders the label stock unusable. Most pressure-sensitive labels use a release liner for this purpose.
Before applying the label to packaging or an end product, the release liner must be removed in order to expose the pressure-sensitive adhesive. A common way in the art to accomplish this task automatically is to remove the pressure-sensitive labels from the web (or sheets of printed label stock) using equipment to peel the printed labels from the release liner and vacuum hold the label in place for application. The release liner as well as the other remaining portions of the web accumulate as waste which must be disposed. Once the labels are removed from the release liner, the labels are applied with light pressure to the packaging or end product by various means known in the art. As mentioned, eliminating the release liner from the pre-printed label stock provides the potential to substantially reduce the cost of producing pressure-sensitive label stock, while also substantially reducing shipping costs for shipping the pre-printed label stock from the production facility to the label printing facility. In addition, the absence of the release liner also reduces waste accumulation at the end user. Thus, the elimination of the release liner can provide a substantial cost savings at both the front end (production and printing) and the back end (label application).
Linerless pressure-sensitive label stock can be manufactured by applying a release coating (normally silicone) on the printing face of the printable web material, i.e., on the side of the web opposite to which the adhesive is applied. The resultant web can then be wound upon itself to form a roll, and then subsequently unrolled without the adhesive sticking to the other layers of the web. Such linerless label stock obviates the need for a separate release liner, however, is known in the art to suffer from several disadvantages. For example, the presence of a silicone release coating on the printable side of the web hinders the ability to print effectively with many inks. The release face of the linerless label typically has little or no porosity and therefore it is difficult for ink or toner to absorb and become anchored. Often, the printing can be wiped off or smeared upon contact.
The concept of a heat-activated, pressure-sensitive label stock has been discussed by others in the art in order to eliminate the need for the silicone release liner over the adhesive or a silicone release coating on the print face. See for example U.S. Pat. No. 4,745,026 by Tsukahara et al., issuing on May 17, 1988 and entitled “Thermal Delayed Tack Sheets”; or, U.S. Pat. No. 6,828,017 by Kugo et al. issuing on Dec. 7, 2004 and entitled “Heat-Sensitive Adhesive Material, Method of Preparing Same, and Method of Using Same.” These patents generally disclose a homogenous heat-activated adhesive layer that includes an adhesive polymer and solid plasticizer particles mixed throughout, as well as selected other constituents.
It is important that a heat-activated, pressure-sensitive adhesive remain completely inert, such that it exhibits no properties of adhesion, until it is heated and activated just prior to its application to the end product. On the other hand, after heat activation, it is important that the resultant adhesive properties be suitable for the application process. Achieving an acceptable balance between these two properties has been proven to be difficult in the past. The above mentioned U.S. Pat. No. 6,828,017 suggests the incorporation of a super cooling improving agent in order to improve the thermal activation chemistry of the adhesive.
Another issue that has been difficult to address in the prior art is identification of a heat-activated adhesive mixture with an appropriate heat-activation temperature. If the required heat activation temperature is too high, the label itself may be damaged or difficult to use. On the other hand, the heat-activation temperature cannot be too low because the adhesive must remain inert under a wide range of conditions in which the label stock is expected to be stored and shipped. For example, the label stock must be able to maintain integrity on a warm day in warehouses or enclosed truck trailers that are not air conditioned.
Another approach to make a linerless pressure-sensitive label is explained in U.S. Pat. No. 5,569,515 by Rice, II et al. issuing on Oct. 29, 1996, and entitled “Printable Linerless Label and Method for Making the Same”. This patent discloses the application of a continuous protective layer of polyethylene or amide wax overcoating the pressure-sensitive adhesive. The protective layer is removed (at least partially) prior to use in order to expose the underlying pressure-sensitive adhesive thereby enabling the label to be subsequently affixed to a surface. The patent explains that the protective coating can be removed by mechanical, chemical or electrochemical means, for example, UV light, ultrasound, corona discharged or knurled roller.